Acrylic acid is a useful compound because polymers and copolymers thereof are used for high water-absorbing materials, paints, adhesives, fibers, and the like. When acrylic acid is produced, though polymerization inhibitor is used since acrylic acid has a high polymerizable property, acrylic acid polymer is still formed. When the polymer exists in acrylic acid, it seems that acrylic acid tends to polymerize more easily. Further, when acrylic acid polymer accumulates, the interruption of the process might be caused by blocking the producing line with the polymers. Moreover, the polymer influences the purity of acrylic acid after purification, and it will be necessary to repeat the purifying operation many times. Accordingly, in order to carry out the efficient purification and consequently the production of acrylic acid, it is needed to exclude acrylic acid polymers in crude acrylic acid as much as possible.
As a method of excluding acrylic acid polymers formed in a process of producing acrylic acid, the method described in Japanese Unexamined Patent Publication No. 2001-129388 can be exemplified. In the publication, such a process is proposed that, in order to exclude solid impurities such as acrylic acid polymers, a processed fluid in a processing column such as a distillation column and a fractionating column is extracted once, filtered with a strainer and the like, and returned again to the processing column. However, polymer cannot be completely removed by such a usual filtrating procedure, since the temperature of the processed fluid is high and low polymerized acrylic acid polymer is dissolved in the processed fluid. Therefore, polymer which could not be removed by a usual filtrating procedure is separated out to develop problems such that the precipitation adheres on devices and causes dirt.
As a method of removing the polymers in crude acrylic acid supplied to purifying process, methods described in Published Japanese Translation of PCT International Publication Nos. 2004-528370 and 2004-528371 can be exemplified. In the publications, adhesive polymers not high in molecular weight are removed by filtering crude acrylic acid melt with a filter. However, since the operation temperature in a condensation process and an absorption process are usually high, the polymers are dissolved in crude acrylic acid. As a result, the polymers existing in crude acrylic acid cannot be completely removed by the aforementioned methods.